Nitrogen passivation, also called “nitridation” is known to be an effective method for reducing the density of interface states at the SiO2/SiC interface. Nitridation techniques, including growth and post-oxidation annealing are described in detail in U.S. patent publication no. 2002/0102358 published Aug. 1, 2002 entitled “Method Of Fabricating An Oxide Layer On A Silicon Carbide Layer Utilizing An Anneal In A Hydrogen Environment”, U.S. patent publication no. 2002/0072247 entitled “Method Of N2O Growth Of An Oxide Layer On A Silicon Carbide Layer” filed Oct. 1, 2001, and U.S. patent application Ser. No. 09/834,283 filed Apr. 12, 2001 entitled “Method Of N2O Annealing An Oxide Layer On A Silicon Carbide Layer” each of which commonly is assigned to the assignee of the present invention and each of which is incorporated herein by reference as if set forth fully herein.
Briefly, nitridation processes introduce nitrogen to the SiO2/SiC interface in sufficient concentrations to passivate or neutralize at least some concentration of “traps” at the interface that would otherwise create unwanted energy states in the layers. “Traps” are localized areas within a semiconductor material that can attract or “trap” free electrical carriers. Traps may be caused by unterminated or dangling electrochemical bonds at the interface of dissimilar materials such as SiO2 and SiC. Traps may also be produced by other types of lattice defects near the interface. Unwanted energy states may impair the electrical properties of material near the SiO2/SiC interface and may reduce the performance of electronic devices that incorporate such interfaces, such as MOSFETs, capacitors and other devices. Traps may also impair the electrical functioning of a device by causing field termination and/or Coulomb scattering.
Local nonuniformities at the SiC surface also may result in an uneven distribution of charge near the interface. In typical SiO2/SiC structures, the uneven distribution of charge may result in a variation in surface potential at the SiC surface having a standard deviation of around 4 kT—a significant fluctuation. In comparison, the surface potential variation in silicon may be less than 1.5 kT. Variation in surface potential may cause undesirable characteristics in electronic devices. For example, variation in surface potential may cause a MOSFET incorporating a SiO2/SiC interface to have a higher and softer turn-on voltage than would otherwise be the case.
Although nitridation by growth and/or post-oxidation annealing using N2O and/or NO has been shown to be successful in reducing the density of interface states (DIT) in SiO2/SiC structures while maintaining acceptable oxide strength, experimental results have shown an upper limit on the reduction of DIT that can be achieved through such techniques. Although the precise reason is not known, it is believed that N2O and NO annealing, while providing nitrogen to passivate the traps, also provides oxygen to the SiO2/SiC interface which causes the interface to grow further into the SiC, which may create additional traps thereby offsetting the benefits of passivation.
Passivation anneals of SiO2/SiC interfaces using ammonia (NH3) as the annealing gas have been investigated by Chung et al. See, e.g. “Effects of anneals in ammonia on the interface trap density near the band edges in 4H-silicon carbide metal-oxide-semiconductor capacitors,” Appl. Phys. Let., Vol. 77, No. 22, pp. 3601–03 (November 2000). Chung et al. produced 40 nm oxide layers on 4H—SiC wafers using standard wet oxidation techniques. The wafers were then annealed in NH3 at a pressure of 1 atm for 2 hours at a temperature between 1050 and 1175° C. Chung et al. found that the interface state density near the conduction band edge was reduced. In “Passivation of the 4H—SiC/SiO2 interface with Nitric Oxide,” Materials Science Forum Vols. 389–393, pp. 967–972 (2002), Chung et al. noted that ammonia appears to be just as effective as NO in reducing the interface state density (DIT) near the conduction band edge. However, Chung et al. noted that the breakdown field strength (i.e. the dielectric strength) was found to be much lower for oxide layers passivated with NH3.